Dual-axis scanning mirror

ABSTRACT

Optical apparatus (64) includes a stator assembly (47), which includes a core (78, 90, 91) containing an air gap and one or more coils (80, 92, 94, 116, 120) including conductive wire wound on the core so as to cause the core to form a magnetic circuit through the air gap in response to an electrical current flowing in the conductive wire. A scanning mirror assembly (45, 83, 85, 130) includes a support structure (68), a base (72), which is mounted to rotate about a first axis relative to the support structure, and a mirror (46), which is mounted to rotate about a second axis relative to the base. At least one rotor (76, 132) includes one or more permanent magnets, which are fixed to the scanning mirror assembly and which are positioned in the air gap so as to move in response to the magnetic circuit. A driver (82) is coupled to generate the electrical current in the one or more coils.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 14/416,296, filed Jan. 22, 2015, in the national phase of PCTPatent Application PCT/IB2013/056101, filed Jul. 25, 2013, which claimsthe benefit of U.S. Provisional Patent Application 61/675,828, filedJul. 26, 2012, and of U.S. Provisional Patent Application 61/835,655,filed Jun. 17, 2013. This patent application is also related to U.S.patent application Ser. No. 13/766,801, filed Feb. 14, 2013, to U.S.patent application Ser. No. 13/798,251, filed Mar. 13, 2013. All of theabove related applications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to optical scanning.

BACKGROUND

Various methods are known in the art for optical 3D mapping, i.e.,generating a 3D profile of the surface of an object by processing anoptical image of the object. This sort of 3D profile is also referred toas a 3D map, depth map or depth image, and 3D mapping is also referredto as depth mapping.

PCT Patent Application PCT/IB2011/053560, which is assigned to theassignee of the present patent application and whose disclosure isincorporated herein by reference, describes apparatus for mapping, whichincludes an illumination module. This module includes a radiationsource, which is configured to emit a beam of radiation, and a scanner,which is configured to receive and scan the beam over a selected angularrange. Illumination optics are configured to project the scanned beam soas to create a pattern of spots extending over a region of interest. Animaging module is configured to capture an image of the pattern that isprojected onto an object in the region of interest. A processor isconfigured to process the image in order to construct athree-dimensional (3D) map of the object.

U.S. Patent Application Publication 2011/0279648, whose disclosure isincorporated herein by reference, describes a method for constructing a3D representation of a subject, which comprises capturing, with acamera, a 2D image of the subject. The method further comprises scanninga modulated illumination beam over the subject to illuminate, one at atime, a plurality of target regions of the subject, and measuring amodulation aspect of light from the illumination beam reflected fromeach of the target regions. A moving-mirror beam scanner is used to scanthe illumination beam, and a photodetector is used to measure themodulation aspect. The method further comprises computing a depth aspectbased on the modulation aspect measured for each of the target regions,and associating the depth aspect with a corresponding pixel of the 2Dimage.

U.S. Pat. No. 8,018,579, whose disclosure is incorporated herein byreference, describes a three-dimensional imaging and display system inwhich user input is optically detected in an imaging volume by measuringthe path length of an amplitude modulated scanning beam as a function ofthe phase shift thereof. Visual image user feedback concerning thedetected user input is presented.

U.S. Pat. No. 7,952,781, whose disclosure is incorporated herein byreference, describes a method of scanning a light beam and a method ofmanufacturing a microelectromechanical system (MEMS), which can beincorporated in a scanning device. In a disclosed embodiment, a rotorassembly having at least one micromirror is formed with a permanentmagnetic material mounted thereon, and a stator assembly has anarrangement of coils for applying a predetermined moment on the at leastone micromirror.

Further MEMS mirror assemblies with magnetic drives are described, forexample, in U.S. Patent Application Publications US 2008/0143196, US2009/0284817, and US 2010/0046054.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide optical scanners with enhanced performance and capabilities.

There is therefore provided, in accordance with an embodiment of thepresent invention, optical apparatus, including a stator assembly, whichincludes a core containing an air gap and one or more coils includingconductive wire wound on the core so as to cause the core to form amagnetic circuit through the air gap in response to an electricalcurrent flowing in the conductive wire. A scanning mirror assemblyincludes a support structure, a base, which is mounted to rotate about afirst axis relative to the support structure, and a mirror, which ismounted to rotate about a second axis relative to the base. At least onerotor includes one or more permanent magnets, which are fixed to thescanning mirror assembly and which are positioned in the air gap so asto move in response to the magnetic circuit. A driver is coupled togenerate the electrical current in the one or more coils at one or morefrequencies selected so that motion of the at least one rotor, inresponse to the magnetic circuit, causes the base to rotate about thefirst axis at a first frequency while causing the mirror to rotate aboutthe second axis at a second frequency.

In one embodiment, the mirror is weighted asymmetrically about at leastthe first axis so as to couple a first rotation of the mirror about thefirst axis to a second rotation of the base about the second axis.Typically, the second frequency is a resonant frequency of rotation ofthe mirror about the second axis, and the driver is coupled to generatethe electrical current at the first frequency.

In some embodiments, the core includes first and second pairs of polepieces, defining the air gap, and the at least one rotor includes firstand second rotors, which are respectively fixed to opposing first andsecond sides of the base and are respectively positioned in the air gapbetween the first and second pairs of the pole pieces. In a disclosedembodiment, the drive circuit is configured to drive the one or morecoils with a first current at the first frequency and a second currentat the second frequency. Typically, the one or more coils include firstcoils wound adjacent to the pole pieces in the first pair and secondcoils wound adjacent to the pole pieces in the second pair, and thedrive circuit is configured to drive the first and second coils in phaseat the first frequency and in opposing phases at the second frequency.

In certain disclosed embodiments, the core includes a tooth, whichprotrudes between the pole pieces and has an upper end that adjoins andcontains the air gap. Typically, the core includes a base, from whichthe pole pieces and the tooth protrude toward the air gap. The one ormore coils may include first coils wound adjacent to the pole pieces anda second coil wound on the tooth, wherein the drive circuit isconfigured to drive the first coils at the first frequency and thesecond coil at the second frequency. In one embodiment, the coreincludes a plurality of fingers, which surround the tooth and protrudetoward the air gap between the tooth and the pole pieces.

In some embodiments, the one or more permanent magnets of the at leastone rotor include first and second permanent magnets fixed to the baseon opposing sides of the mirror. In one embodiment, the first and secondpermanent magnets have a rectangular shape. Additionally oralternatively, each of the first and second permanent magnets includesrespective upper and lower pieces, which are mounted on opposingsurfaces of the base so that a center of mass of the first and secondpermanent magnets is located on the first axis.

Further additionally or alternatively, the one or more permanent magnetsof the at least one rotor may include at least a third permanent magnetfixed to the mirror. The at least third permanent magnet may be recessedwithin a surface of the mirror.

In disclosed embodiments, the scanning mirror assembly includes asilicon wafer formed as a microelectromechanical systems (MEMS) device,which includes first spindles, etched from the silicon wafer, connectingthe base to the substrate along the first axis and second spindles,etched from the silicon wafer, connecting the mirror to the base alongthe second axis. Typically, the second spindles are formed so that themirror rotates resonantly about the second axis at the second frequency,while the first spindles are formed so that rotation of the base aboutthe first spindles is a non-resonant rotation. The wafer may be thinnedin a vicinity of the first spindles so as to increase a flexibility ofthe first spindles.

In a disclosed embodiment, the apparatus includes a transmitter, whichis configured to direct pulses of light to reflect from the mirror whilethe mirror and the base rotate, whereby the light is scanned over ascene. A receiver is configured to receive the pulses of the lightreflected from the scene so as to measure a time of flight of thepulses.

There is also provided, in accordance with an embodiment of the presentinvention, optical apparatus, which includes a mirror assembly,including a mirror, which is mounted to rotate about an axis relative toa support structure. A capacitive sensor includes at least first andsecond plates, which are positioned in proximity to the mirror onopposite sides of the axis and are angled relative to a plane of thesupport structure such that the plates are closest to the plane in alocation adjacent to the axis and slope away from the plane at locationsfarther from the axis.

In a disclosed embodiment, the mirror is mounted so as to rotate aboutfirst and second axes, which are mutually perpendicular, and the atleast first and second plates of the capacitive sensor include fourplates, which together define a pyramidal shape, having a peak adjacentto a center point at which the axes intersect.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for scanning, which includes providing astator assembly, which includes a core containing an air gap, and one ormore coils including conductive wire wound on the core so as to causethe core to form a magnetic circuit through the air gap in response toan electrical current flowing in the conductive wire. A scanning mirrorassembly is provided, including a support structure, a base, which ismounted to rotate about a first axis relative to the support structure,and a mirror, which is mounted to rotate about a second axis relative tothe base. At least one rotor, which includes one or more permanentmagnets, is fixed to the scanning mirror assembly. The scanning mirrorassembly is mounted on the stator assembly so that the one or morepermanent magnets are positioned in the air gap so as to move inresponse to the magnetic circuit. The one or more coils are driven withan electrical current at one or more frequencies selected so that motionof the at least one rotor, in response to the magnetic circuit, causesthe base to rotate about the first axis at a first frequency whilecausing the mirror to rotate about the second axis at a secondfrequency.

There is further provided, in accordance with an embodiment of thepresent invention, a method for monitoring, which includes mounting amirror to rotate about an axis relative to a support structure. At leastfirst and second plates of a capacitive sensor are positioned inproximity to the mirror on opposite sides of the axis, while angling theplates relative to a plane of the support structure such that the platesare closest to the plane in a location adjacent to the axis and slopeaway from the plane at locations farther from the axis. Changes in acapacitance between the plates and the mirror are measured so as tomonitor rotation of the mirror.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, pictorial illustration of an optical scanninghead, in accordance with an embodiment of the present invention;

FIG. 2 is a schematic, pictorial illustration of a MEMS scanner, inaccordance with an embodiment of the present invention;

FIG. 3A is a schematic bottom view of a micromirror assembly, inaccordance with an embodiment of the present invention;

FIG. 3B is a schematic frontal view of a part of a micromirror assembly,in accordance with another embodiment of the present invention;

FIG. 4 is a schematic, pictorial illustration of a MEMS scanner, seenfrom below, in accordance with an embodiment of the present invention;

FIG. 5 is a schematic, pictorial illustration showing stators and rotorsof a MEMS scanner, in accordance with an embodiment of the presentinvention;

FIG. 6 is a schematic, pictorial illustration showing details of astator and rotor of a MEMS scanner, in accordance with anotherembodiment of the present invention;

FIG. 7 is a schematic side view of one of the stators of FIG. 5,including arrows indicating the magnetic vector field generated by thestator in accordance with an embodiment of the present invention;

FIGS. 8-10 are schematic plots of current waveforms that are used todrive a stator of a MEMS scanner, in accordance with an embodiment ofthe present invention;

FIGS. 11A and 11B are schematic side views of the stator and rotor ofFIG. 6, showing current used in driving the stator and field directionsgenerated as a result, in accordance with an embodiment of the presentinvention;

FIG. 12 is a schematic, cutaway view of a scanning micromirror, inaccordance with an embodiment of the present invention;

FIG. 13 is a schematic, pictorial illustration of a stator of a MEMSscanner, in accordance with an alternative embodiment of the presentinvention;

FIG. 14A is a schematic side view of a part of the stator of FIG. 13,illustrating a magnetic field generated by the stator, in accordancewith an embodiment of the present invention;

FIG. 14B is a schematic side view of a stator, in accordance withanother embodiment of the present invention;

FIG. 15 is a schematic bottom view of a micromirror assembly, inaccordance with a further embodiment of the present invention;

FIG. 16 is a schematic detail view of a MEMS spindle, in accordance withan embodiment of the present invention; and

FIG. 17 is a schematic bottom view of a micromirror assembly with acapacitive sensor, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

The above-mentioned U.S. patent application Ser. No. 13/766,801describes depth engines that generate 3D mapping data by measuring thetime of flight of a scanning beam. A light transmitter, such as a laser,directs short pulses of light toward a scanning mirror, which scans thelight beam over a scene of interest. A receiver, such as a sensitive,high-speed photodiode (for example, an avalanche photodiode) receiveslight returned from the scene via the same scanning mirror. Processingcircuitry measures the time delay between the transmitted and receivedlight pulses at each point in the scan. This delay is indicative of thedistance traveled by the light beam, and hence of the depth of theobject at the point. The processing circuitry uses the depth data thusextracted in producing a 3D map of the scene.

When using this sort of scanning system for 3D mapping, as well as otherscanning applications, it is desirable that the mirror scan mechanicallyabout at least one axis at high frequency (for example, in the range of2-30 kHz) and over large angles (typically ±10-25°). The scan rangeabout the second scan axis may be even larger, but the scan frequency istypically much lower (for example, in the range of 15-100 Hz). The twoscan directions are coordinated to generate a raster pattern that coversthe region being scanned. The above-mentioned patent applicationsdescribe gimbaled micro-mirror mounts and magnetic driving arrangementsthat can be used in this sort of raster generation.

Embodiments of the present invention that are described herein provideenhancements to these magnetic driving arrangements that areparticularly useful in achieving efficient scanning over a wide angularrange in the high-frequency scan direction. In the disclosedembodiments, a micromirror is mounted on a miniature gimbaled base, sothat the base rotates relative to a support structure in thelow-frequency (slow) scan direction, while the micromirror itselfrotates relative to the base in the high-frequency (fast) scandirection. (The term “micromirror” is used herein simply to refer tovery small mirrors, which are typically no more than a few millimetersacross, although it may be possible to apply the principles of thepresent invention to larger mirrors.) The same magnetic drive can beused to power both the fast and slow scans. Energy may be coupled intothe fast scan direction by mechanical coupling between the mirror andthe base. Additionally or alternatively, the magnetic field of the drivemay be dynamically shaped, by providing appropriate driving currents, inorder to exert alternating magnetic forces on the mirror and the base atdifferent frequencies and in different directions so as to give thedesired pattern of motion.

System Description

FIG. 1 schematically illustrates elements of an optical head 40 that isused in the system described in the above-mentioned U.S. patentapplication Ser. No. 13/766,801. The performance of this optical headmay be enhanced by incorporation of a gimbaled mirror with magneticdrive as described below. A transmitter 44 emits pulses of light towarda polarizing beamsplitter 60. Typically, only a small area of thebeamsplitter, directly in the light path of transmitter 44, is coatedfor reflection, while the remainder of the beamsplitter is fullytransparent in the transmitted wavelength range (or even anti-reflectioncoated for it) to permit returned light to pass through to a receiver48. The light from transmitter 44 reflects off beamsplitter 60 and thena folding mirror 62 toward a scanning micromirror 46. The area of themicromirror may be about 12 mm², for example. A MEMS scanner 64 scansthe micromirror in X-and Y-directions with the desired scan frequencyand amplitude. Details of the micromirror and scanner are shown in thefigures that follow.

Light pulses returned from the scene strike micromirror 46, whichreflects the light via folding mirror 62 through beamsplitter 60.Receiver 48 senses the returned light pulses and generates correspondingelectrical pulses. To enhance sensitivity of detection, it is desirablethat the active mirror size, the overall area of beamsplitter 60 and theaperture of receiver 48 be considerably larger than the area of thetransmitted beam. To limit the amount of unwanted ambient light thatreaches receiver 48, a bandpass filter (not shown) may be incorporatedin the receiver path, possibly on the same substrate as beamsplitter 60.

The specific mechanical and optical designs of the optical head shown inFIG. 1 are described here by way of example, and alternative designsimplementing similar principles are considered to be within the scope ofthe present invention.

FIG. 2 is a schematic, pictorial illustration of MEMS scanner 64, inaccordance with an embodiment of the present invention. This scanner isproduced and operates on principles similar to those described in theabove-mentioned U.S. Pat. No. 7,952,781, but enables two-dimensionalscanning of a single micromirror 46. (The pictured arrangement may beused, mutatis mutandis, to drive an array of micromirrors, as describedin the above-mentioned U.S. patent application Ser. No. 13/798,251.) Amicromirror assembly 45, which includes micromirror 46, is produced bysuitably etching a semiconductor substrate 68 to separate themicromirror from a base 72 (also referred to as a gimbal), and toseparate the base from the remaining substrate 68, which serves as asupport structure. After etching, micromirror 46 (to which a suitablereflective coating is applied) is able to rotate about the X-axis (toscan the reflected spot in the Y-direction) relative to support 72 onspindles 70, while base 72 rotates about the Y-axis (to scan the spot inthe X-direction) relative to substrate 68 on spindles 74.

Micromirror 46 and base 72 are mounted on a pair of rotors 76, whichcomprise permanent magnets. (Only one of the rotors is visible in thisfigure.) Rotors 76 are suspended in respective air gaps between the polepieces of magnetic cores 78 in a stator assembly 47 (also referred tosimply as the stator of scanner 64). Cores 78 are wound with respectivecoils 80 of conductive wire, thus creating an electromagnetic statorassembly. Although a single coil per core is shown in FIG. 2 for thesake of simplicity, two or more coils may alternatively be wound on eachcore; coils may be wound at different places on the cores; and differentcore shapes may also be used, as shown in the figures that follow.

A drive circuit 82 (referred to for short as a driver) drives anelectrical current through coils 80 so as to generate a magnetic circuitthrough cores 78 that passes through the air gaps. Typically, drivecircuit 82 comprises a frequency generator, which generates electricalsignals at the desired frequency or frequencies, along with suitableamplifiers to provide the desired current levels to the coils. Themagnetic field generated in the air gaps of the coils interacts with themagnetization of rotors 76 and thus causes the rotors to move within theair gaps.

Specifically, in this embodiment, coils 80 may be driven withhigh-frequency differential currents so as to cause micromirror 46 torotate resonantly back and forth about spindles 70 at high frequency(typically in the range of 2-30 kHz, as noted above). This resonantrotation generates the high-speed Y-direction raster scan of the outputbeam from optical head 40. At the same time, coils 80 are driventogether at lower frequency to drive the X-direction scan by rotation ofbase 72 about spindles 74 through the desired scan range. (In accordancewith the corresponding speeds of rotations about the axes, the X-axis inthe embodiments described herein is also referred to as the “fast axis,”while the Y-axis is referred to as the “slow axis.”) Alternatively,other stator configurations and drive schemes, some of which areillustrated in the figures that follow, may be used for these purposes.The X-and Y-rotations together generate the overall raster scan patternof micromirror 46.

Assembly of optical head 40 from discrete optical and mechanicalcomponents, as shown in FIG. 1, requires precise alignment and can becostly. In alternative embodiments, all parts requiring preciseplacement and alignment (such as the light transmitter, receiver, andassociated optics) may be combined in a single integrated package on asilicon optical bench (SiOB). This approach can save costs and may makethe optical head easier to handle. Various alternative designs of thesesorts are shown in the above-mentioned U.S. patent application Ser. No.13/766,801.

Scan Coupling Using Mechanical Asymmetry

FIG. 3A is a schematic bottom view of a micromirror assembly 83, showinga scheme for mechanically driving the high-speed Y-direction scan ofmicromirror 46, in accordance with an embodiment of the presentinvention. Magnetic rotors 76 appear as rectangular structures in thisfigure, as opposed to the cylindrical rotor shown in FIG. 2, but performthe identical function. Spindles 70 in FIG. 3A are aligned with theX-axis as shown in FIG. 2, while spindles 74, aligned with the Y-axis,are hidden in FIG. 3A by rotors 76.

A pair of weights 84 are attached to the bottom of micromirror 46 atopposite corners. The weights may comprise any suitable material and maybe fastened (using a suitable glue, for example) to the micromirror inthe appropriate locations. Alternatively, the weight asymmetry can becreated by etching away appropriate portions of the backside of themirror, using standard etching processes, for example, such as DRIE orwet etching. The purpose of the weights is to introduce mechanicalasymmetry about the axes of rotation of the micromirror. The particularshape and locations of the weights in FIG. 3A are thus shown solely byway of example, and any design that provides a suitable asymmetricweight distribution about the axes may similarly be used.

The asymmetric weight distribution induces mechanical coupling betweenthe axes of rotation by changing the axis of rotational inertia ofmicromirror 46. The new axis of inertia thus created for the fast scandirection is not precisely perpendicular to the axis of the slow scan.Therefore, as base 72 rotates about the Y-axis due to the operation ofthe magnetic drive, weights 84 will cause micromirror 46 to wobble aboutthe X-axis on spindles 70. The mechanical design of the micromirror,spindles and weights is chosen to give the desired scan frequency andamplitude under these conditions. Adding a high-frequency electricaldrive to the micromirror at the resonant frequency for the fast scanrotation will cause significant mechanical energy to be transferred tothis scan direction, thus generating the desired two-dimensional scan ofthe micromirror.

FIG. 3B is a schematic frontal view of a part of a micromirror assembly85, in accordance with another embodiment of the present invention. Inthis case, an asymmetric weight distribution is formed by appropriatelyetching the shape of micromirror 46 in the course of thephotolithographic production process so as to provide an asymmetricweight distribution. The shaping of the mirror in this case provides thedesired mechanical coupling between the fast and slow axes of rotation.

Electrical and Magnetic Drive Schemes

The figures described below illustrate various schemes that may be usedto drive dual-axis scanning of a micromirror, in accordance with severaldifferent embodiments of the present invention. For convenience andclarity, these embodiments are described with reference to micromirror46 and the associated MEMS structure that is presented above. Theseprinciples of these embodiments, however, may likewise be applied,mutatis mutandis, to dual-axis scanning mirrors of other sorts, such assome of the devices that are described in the references cited above inthe Background section.

FIG. 4 is a schematic bottom view of MEMS scanner 64, in accordance withanother embodiment of the present invention. In this embodiment, a pairof magnetic strips 86, 88 are fixed to the underside of micromirror 46.The strips have opposing polarities, as indicated by the arrows in thefigure, with the north pole of strip 86 oriented downward and that ofstrip 88 oriented upward. It can be advantageous in this sort ofembodiment to recess strips 86, 88 within the volume of micromirror sothat the center of mass of the micromirror is close to the axis definedby spindles 70. An embodiment of this sort is shown in FIG. 15.

The currents driving coils 80 include a differential component at theresonant frequency of the micromirror, i.e., the waveforms driving thecoils at this frequency are 180° out of phase with one another. Thisdifferential component gives rise to a magnetic field component alongthe Y-axis between cores 78, which is inhomogeneous in the X-directionand alternates in direction at the driving frequency. In other words, ata given point in time, the field may point in the positive Y-directionat magnetic strip 86 and in the negative Y-direction at magnetic strip88, with point directions alternating at the resonant frequency.Interaction of this magnetic field with the magnetization of magneticstrips 86, 88 gives rise to an alternating rotational force exerted onmicromirror 46 about spindles 70, thus causing the micromirror to rotateas shown in the figure.

More generally, the coil and driving current configurations may beadapted by other means to create net forces and torques in the intendeddirection. Magnetic or ferromagnetic material may be applied to themirror itself in other geometries (other than the specific geometriesthat are shown in the figures), while the driving force is created bystationary coils. There may be one or more such magnets, with anysuitable polarities, or ferromagnetic material (with no inherentpolarity). The driving electromagnetic fields can be adjusted in eachcase to create driving forces in the appropriate directions, as will beapparent to those skilled in the art upon reading the present patentapplication. All such alternative configurations of magnets and drivesare considered to be within the scope of the present invention.

FIG. 5 is a schematic pictorial view showing elements of MEMS scanner 64in accordance with yet another embodiment of the present invention. Inthis embodiment, micromirror 46 itself is omitted for visual clarity,and its location is indicated by rotors 76A and 76B, which are attachedto the underside of gimbaled base 72 of the micromirror as shown in thepreceding figures. The stator assembly of the MEMS scanner comprises twocores 90, 91, each having a pair of pole pieces that define an air gapin which the corresponding rotor 76A or 76B is suspended. (The term“pole piece” is used in the present description and in the claims in theconventional sense, to denote the part of the magnetic core that isadjacent to the air gap.) Each core 90, 91 is wound with two coils 92,94 on opposing sides of the air gap of the core. The effect of this sortof arrangement, in generating magnetic circuits that drive the motion ofthe rotors, is illustrated in the figures that follow.

FIG. 6 schematically shows details of structure of one stator core 90and rotor 76 that are used in a magnetic drive, in accordance withanother embodiment of the present invention. This is one of a pair ofrotors and cores, as in the preceding embodiment. The coils andmicromirrors are omitted from the figure in order to give a clear viewof the shape of the core, including the air gap between the poles and anaddition pole in the form of a protruding “tooth” 100, whose upper endadjoins and contains the air gap from below. The function of this toothis explained below.

The rectangular profile of rotor 76 in this and other embodiments can beadvantageous, inter alia, in that it gives rise to a rotational momentof the rotor about the Y-axis that grows as the angle of rotation growsfrom the central position shown in the figure, due to attraction by thepole pieces of the stator core that are located to either side of therotor. This rotational moment counteracts the spring force of spindles74, which increases as the rotor turns about the spindle (Y) axis, andthus reduces the force that must be exerted in rotating the mirror aboutspindles 74. Alternatively, other geometrical designs of the rotors andthe stator core may be used to engender the desired rotationalcharacteristics.

FIG. 7 is a schematic side view of core 90, as shown pictorially in FIG.5, in accordance with an embodiment of the present invention. In thisembodiment, core 90 comprises an enlarged base 96, with coils 92 and 94wound on the core above the base. Core 91 (FIG. 5) will have a similarform and behavior. FIG. 7 also includes arrows indicating the flux ofthe magnetic circuit in the plane of the figure (including the flux inthe air gap between the poles of core 90) when the coils are driven withappropriate currents. Alternatively, however, other core and coilconfigurations may be used and are considered to be within the scope ofthe present invention, as noted earlier.

The high-frequency magnetic field generated in and around core 90, asindicated by the arrows, includes a vertical (Z-direction) component inthe air gap between the poles of the core.

This field component alternates in direction at the frequency of thecurrents driving coils 92 and 94. Coils 92 and 94 on core 91 are drivenwith opposite phase to the currents in the counterpart coils 92, 94 ofcore 90, so that the Z-direction field components at any moment in theair gaps of cores 90 and 91 are likewise opposite. Thus, the field willpush rotor 76A upward while pushing rotor 76B downward, and vice versa,alternating at the drive frequency of the high-frequency waveform. Theseopposing Z-direction movements of the rotors cause micromirror 46 torotate on spindle 70 at this same frequency, which is typically chosento be the resonant frequency of the micromirror. (More precisely, interms of Newton's Laws, inertia causes micromirror 46 to tend to remainin place while base 72 moves, thereby creating elastic energy transferfrom the rotors to the micromirror, through the torsion arms of spindles70; this principle is behind the operation of a number of theembodiments described herein.) At the same time, the low-frequencycurrent component, driven in phase through all of coils 92 and 94, i.e.,with the current flowing in the same direction in all the coils, givesrise to an alternating X-direction magnetic field within the air gaps,thus causing rotors 76A, 76B to rotate about the Y-axis and in thismanner rotate base 72.

FIGS. 8-10 schematically illustrate typical current waveforms that areused to drive coils 92 and 94, in accordance with an embodiment of thepresent invention. As noted above, all of the coils are driven in phasewith a low-frequency waveform, such as the sawtooth shown in FIG. 7. Thecoils are driven in opposite phases by a high-frequency waveform, suchas the sinusoid shown in FIG. 8. Coils 92 and 94 on each core are driven180° out of phase relative to one another; and each coil 92, 94 on core90 is driven 180° out of phase relative to its counterpart coil 92, 94on core 91. Each coil is thus driven by the superposition of waveformsthat is shown in FIG. 9, with variations in the phase of thehigh-frequency component from coil to coil, thus generating rotation ofthe micromirror about two axes using a stationary set of coils.

The currents applied to coils 92 and 94 (and similarly, the currentsapplied in other embodiments of the present invention) may be generatedand controlled using any suitable technique that is known in the art,including both open-loop and closed-loop controls. In the lattercategory, drive circuit 82 may receive feedback regarding the amplitudeand/or frequency of the mirror scan and may control the currentaccordingly. The novel capacitive sensing scheme that is shown in FIG.17 may be used for this purpose, for example, but other sorts of sensingschemes that are known in the art may alternatively be used, such as theclosed-loop schemes using sensors of various sorts that are described inthe above-mentioned U.S. Pat. No. 7,952,781.

FIGS. 11A and 11B are schematic side views of the stator core 90 androtor 76 of FIG. 6, showing currents used in driving the stator andfield directions generated as a result, in accordance with an embodimentof the present invention. As illustrated by arrows 101 and 102 in FIG.11A, the low-frequency (slow axis) currents flow in phase through coils92 and 94 on both stator cores, giving a horizontal (X-direction)magnetic flux through the air gap, as indicated by an arrow 103. ThisX-direction field alternates at the frequency of the in-phase currentand causes both rotors to rotate about the Y-axis.

On the other hand, as shown in FIG. 11B, the high-frequency (fast axis)currents flow through coils 92 and 94 in anti-phase, as indicated byarrows 104 and 105. These currents give rise to an alternating vertical(Z-direction) magnetic flux, indicated by arrows 106, which causesrotors 76A and 76B to move in opposite directions along the Z-axis andthus drive the high-frequency scan of the micromirror. Protruding tooth100 below the air gap enhances the this field and hence the drivingforce for fast rotation about the X-axis.

FIG. 12 is a schematic, cutaway view of scanning micromirror 46 andassociated parts of a micromirror assembly, in accordance with anembodiment of the present invention. This figure illustrates how thecurrents and field that are shown in FIG. 11B drive the fast rotation ofmicromirror 46. The alternating Z-direction flux in the air gap ofstator core 90 causes rotors 76A and 76B to vibrate up and down, inopposing phases, along the Z-axis at the frequency of the alternatingcurrent. As a result, spindles 74 flex in opposing directions, causingbase 72 to vibrate, as well, as shown in the figure. Because thefrequency of vibration is equal or close to the resonant rotationalfrequency of micromirror 46 about spindles 70, the twist of the spindlesdue to vibration of base 72 causes the micromirror to rotate back andforth about the X-axis with high amplitude.

FIG. 13 is a schematic, pictorial illustration of a stator 110, inaccordance with a further embodiment of the present invention. Stator110 includes central tooth 100, which in this embodiment is wound withits own coil 120. This coil is energized so as to drive the fast-axisrotation of micromirror 46. Stator 110 is based on a magnetic core,which comprises two pairs of posts 112 on a base 114, together withtooth 100. Pole pieces 118 at the tops of posts 112, along with tooth100, define the air gap in which the rotors of the micromirror assemblyare situated during operation. Coils 116 on posts 112 are driven inphase with alternating currents at the frequency of slow-axis rotationof gimbaled base 72 and thus turn the rotors in the areas of the air gapbetween pole pieces 118.

Coil 120, however, is driven at the much higher frequency of (resonant)fast-axis rotation of the micromirror. As a result, tooth 100 generatesa high-frequency magnetic field, which interacts with a magnet ormagnets that are mounted on micromirror 46 itself and thus causes themicromirror to rotate about spindles 70 relative to base 72. FIG. 15shows a configuration of micromirror 46 that may be used in thiscontext, with a suitable magnet mounted on the back side of themicromirror.

FIG. 14A is a schematic side view of a part of stator 110, absent coils116, illustrating the magnetic circuit generated in the air gap of thestator due to the current in coil 120 on tooth 100, in accordance withan embodiment of the present invention. The marks in the figure in thespace between posts 112 represent the direction and magnitude of themagnetic field at each point in the space.

The geometrical arrangement of tooth 100 and posts 112 causes themagnetic field around the tooth to have the general form of a“fountain,” with the lines of magnetic force “spraying” outward from thetooth toward the posts. As the direction of this field alternates, dueto the alternating current driven through coil 120, the direction of themagnetic force exerted on micromirror 46 about the axis of spindles 70likewise alternates, thus causing the micromirror to rotate at thefrequency of the alternating current. The upward (Z-direction) fieldthat is shown in this figure interacts with the Y-directionmagnetization of a magnet mounted on micromirror 46 (as shown in FIG.15) to drive rotation of the micromirror about the fast (X-direction)axis defined by spindles 70.

FIG. 14B is a schematic side view of a stator 125, in accordance withanother embodiment of the present invention. The structure of stator 125is similar to that of stator 110, with the addition of two or moreauxiliary fingers 126 positioned on base 114 around tooth 100. Fingers126 further bound and define the air gap of the stator core, locatedbetween poles 118, and have the effect to constraining magnetic fieldlines 128 so that the vertical (Z-direction) component of the field inthe air gap is enhanced. As a result, the amplitude of the fast-axisrotation of the micromirror, relative to the driving current, isenhanced, as well.

FIG. 15 schematically illustrates a silicon scanning micromirrorassembly 130, in accordance with an embodiment of the present invention.Assembly 130 is seen from the bottom in this figure, i.e., from the sidethat is adjacent to the stator (which may have the form of stator 110,for example). The reflective (upper) side of micromirror 46 faces intothe page in this view. Assembly 130 may be driven by the magnetic forcesof stator 110, for example, in the manner described above.

Micromirror 46 in assembly 130 is connected to gimbaled base 72 byspindles 70, while base 72 is connected by spindles 74 to substrate 68,as in the preceding embodiments. In contrast to the simplifiedillustrations of the embodiments shown in FIGS. 2 and 3, however,magnetic rotors 132 in assembly 130 each comprise upper and lower pieces138 and 140, which are attached directly to both sides of base 72. Thus,micromirror 46 is balanced between rotors 132, rather than mounted abovethem. (The embodiments described above could also be implemented in asimilar, balanced fashion.) As a result, the plane of the micromirrorand its base will be located between the two cores of the statorassembly, rather than above the cores as in the preceding embodiments(as seen in FIG. 2, for example). This balanced configuration of themicromirror is advantageous in terms of mechanical stability, as theaxis of rotation of base 72 that is defined by spindles 74 passesthrough the center of mass of assembly 130.

A magnet 134 is fixed to the lower side of micromirror 46 and interactswith the alternating magnetic field generated by tooth 100 of the stator(as illustrated in FIG. 14A). Magnet 134 is polarized along the Y-axis,as indicated by the arrow in the figure. The interaction of thismagnetization with the Z-direction field shown in FIG. 14A provides theforce that causes micromirror 46 to rotate about the axis of spindles 70(the X-axis, i.e., the fast axis).

To enhance the mechanical stability of assembly 130, magnet 134 ismounted in an indentation 136 that is formed in the back side ofmicromirror 46. The indentation may be formed, for example, by wetetching or by deep reactive ion etching (DRIE) of the silicon.Consequently, the axis of rotation of the micromirror that is defined byspindles 70 passes close to the center of mass of the micromirror withmagnet 134 mounted in the indentation. Although it may not be possibleto place the center of mass and the center of rotational inertiaprecisely on the axis of rotation, reducing the distance between thesepoints in the manner described above is still useful in improving thebalance and dynamic stability of the rotating micromirror.

Enhancement and Monitoring of Mirror Rotation

FIG. 16 is a schematic detail view of spindle 74, in accordance with anembodiment of the present invention. To reduce the force that must beapplied in rotating base 72 relative to substrate 68 (which is typicallynot a resonant rotation), it is desirable that spindle 74 be asrotationally flexible as possible. Therefore, the silicon wafer in thearea of base 72 surrounding spindle 74 is thinned, by a wet etchingprocess, for example. The shape of the spindle is then defined and cutout of the thinned wafer by photolithography. To strengthen the spindleagainst breakage, while still maintaining the desired rotationalflexibility, the gaps surrounding spindle 74 may be filled with asuitable filler material, such as a flexible polymer, as described inU.S. Provisional Patent Application 61/781,086, which is incorporatedherein by reference. Using this approach, it is possible to decouple thedesign considerations of slow spindles 74 from those of fast spindles70.

The process that is used to thin the wafer can be critical in designs ofthe sort shown in FIG. 16. Some processes may tend to cause micro-cracksand roughness in the silicon wafer, which may weaken the spindles; andit is therefore undesirable to thin the spindles using such processes.The wet etching process, however, leaves smooth surfaces, which arebeneficial in producing robust spindles with a reduced likelihood ofcracking.

FIG. 17 schematically illustrates a capacitive sensor 150 that is usedto sense the rotation of micromirror 46, in accordance with anembodiment of the present invention. Sensor 150 comprises sensing platesin the form of four quadrants 152, 153, 154 and 155, which are typicallymade from a conductive material. As base 72 rotates about spindles 74(i.e., about the slow axis), as shown in the figure, the capacitancebetween micromirror 46 and quadrants 154 and 155 increases, while thecapacitance between the micromirror and quadrants 152 and 153 decreases.When the base rotates back toward the opposite extreme of its motion,the capacitance between the micromirror and quadrants 152 and 153increases, while that between the micromirror and quadrants 154 and 155decreases.

A controller (not shown in the figures) measures these changes incapacitance continuously, by methods of measurement that are known inthe art. Based on the changes in capacitance, the controller is able tomonitor the frequency and amplitude (i.e., angular range) of rotation ofbase 72 about the slow axis.

By the same token, rotation of micromirror 42 about spindles 70 willcause changes in the capacitance between the micromirror and quadrants152 and 154 relative to quadrants 153 and 155; and these changes maylikewise be measured to monitor the rotation of the micromirror aboutthe fast axis.

Although it is possible to mount the elements of a capacitive sensor forthis purpose in a plane that is parallel to substrate 68, this mountingscheme may limit the range of motion of micromirror 46 unless the sensorelements are mounted far away from the micromirror (in which case thecapacitance, and hence the useful signal for measuring rotation, isdrastically reduced). To overcome this limitation, quadrants 152, 153,154 and 155 are angled, as shown in FIG. 17, so that the quadrants areclosest to the plane of substrate 68 in the area of the axis of rotationof the micromirror and slope away from this plane at locations fartherfrom the axis. In the configuration shown in the figure, quadrants 152and 153 are mutually parallel, as are quadrants 154 and 155, thusdefining a “roof” with its ridgeline near the axis of spindles 74.Although sensor 150 is illustrated in this embodiment in the context ofa dual-axis scanning micromirror assembly, this sort of roof-shapedsensor configuration may likewise be used in sensing the rotation ofsingle-axis scanners.

Alternatively, quadrants 152 and 153 may be angled relative to oneanother, as well, and likewise quadrants 154 and 155, so that the fourquadrants together define a pyramidal shape, with its peak near thecenter of the micromirror, where the axes of rotation intersect.

Although capacitive sensor 150 is described, for the sake of clarity,with reference to micromirror 46, sensors of this sort may similarly beapplied, mutatis mutandis, in sensing and tracking the motion ofscanning mirrors of other types, such as those described in thereferences cited above in the Background section. By the same token,other inventive features described above with regard to techniques fordriving a scanning mirror may similarly be applied to other mirrordesigns.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

The invention claimed is:
 1. Optical apparatus, comprising: a statorassembly, which comprises: a core comprising a pair of pole pieces,defining an air gap, and a tooth, which protrudes between the polepieces and has an upper end that adjoins and contains the air gap; andone or more coils comprising conductive wire wound on the core so as tocause the core to form a magnetic circuit through the air gap inresponse to an electrical current flowing in the conductive wire; ascanning mirror assembly, comprising: a support structure; a base, whichis mounted to rotate about a first axis relative to the supportstructure; and a mirror, which is mounted to rotate about a second axisrelative to the base; at least one rotor, which comprises one or morepermanent magnets, which are fixed to the scanning mirror assembly andwhich are positioned in the air gap so as to move in response to themagnetic circuit; and a driver, which is coupled to generate theelectrical current in the one or more coils at one or more frequenciesselected so that motion of the at least one rotor, in response to themagnetic circuit, causes the base to rotate about the first axis at afirst frequency while causing the mirror to rotate about the second axisat a second frequency.
 2. The apparatus according to claim 1, whereinthe mirror is weighted asymmetrically about at least the second axis soas to couple a second rotation of the mirror about the second axis to afirst rotation of the base about the first axis.
 3. The apparatusaccording to claim 2, wherein the second frequency is a resonantfrequency of rotation of the mirror about the second axis, and whereinthe driver is coupled to generate the electrical current at the firstfrequency.
 4. The apparatus according to claim 1, wherein the corecomprises first and second pairs of pole pieces, defining the air gap,and wherein the at least one rotor comprises first and second rotors,which are respectively fixed to opposing first and second sides of thebase and are respectively positioned in the air gap between the firstand second pairs of the pole pieces.
 5. The apparatus according to claim4, wherein the driver is configured to drive the one or more coils witha first current at the first frequency and a second current at thesecond frequency.
 6. The apparatus according to claim 5, wherein the oneor more coils comprise first coils wound adjacent to the pole pieces inthe first pair and second coils wound adjacent to the pole pieces in thesecond pair, and wherein the drive circuit is configured to drive thefirst and second coils in phase at the first frequency and in opposingphases at the second frequency.
 7. The apparatus according to claim 1,wherein the core comprises a core base, from which the pole pieces andthe tooth protrude toward the air gap.
 8. The apparatus according toclaim 1, wherein the one or more coils comprise first coils woundadjacent to the pole pieces and a second coil wound on the tooth, andwherein the driver is configured to drive the first coils at the firstfrequency and the second coil at the second frequency.
 9. The apparatusaccording to claim 8, wherein the core comprises a plurality of fingers,which surround the tooth and protrude toward the air gap between thetooth and the pole pieces.
 10. The apparatus according to claim 1,wherein the one or more permanent magnets of the at least one rotorcomprise first and second permanent magnets fixed to the base onopposing sides of the mirror.
 11. The apparatus according to claim 10,wherein the first and second permanent magnets have a rectangular shape.12. The apparatus according to claim 10, wherein each of the first andsecond permanent magnets comprises respective upper and lower pieces,which are mounted on opposing surfaces of the base so that a center ofmass of the first and second permanent magnets is located on the firstaxis.
 13. The apparatus according to claim 10, wherein the one or morepermanent magnets of the at least one rotor comprise at least a thirdpermanent magnet fixed to the mirror.
 14. The apparatus according toclaim 13, wherein the at least third permanent magnet is recessed withina surface of the mirror.
 15. The apparatus according to claim 1, whereinthe scanning mirror assembly comprises a silicon wafer formed as amicroelectromechanical systems (MEMS) device, which comprises firstspindles, etched from the silicon wafer, connecting the base to thesupport structure along the first axis and second spindles, etched fromthe silicon wafer, connecting the mirror to the base along the secondaxis.
 16. The apparatus according to claim 15, wherein the secondspindles are formed so that the mirror rotates resonantly about thesecond axis at the second frequency.
 17. The apparatus according toclaim 15, wherein the first spindles are formed so that rotation of thebase about the first spindles is a non-resonant rotation, and whereinthe wafer is thinned in a vicinity of the first spindles so as toincrease a flexibility of the first spindles.
 18. The apparatusaccording to claim 1, and comprising a capacitive sensor, comprising atleast first and second plates, which are positioned in proximity to themirror on opposite sides of the first axis and are angled relative to aplane of the support structure such that the plates are closest to theplane in a location adjacent to the first axis and slope away from theplane at locations farther from the first axis.
 19. The apparatusaccording to claim 1, and comprising: a transmitter, which is configuredto direct pulses of light to reflect from the mirror while the mirrorand the base rotate, whereby the light is scanned over a scene; and areceiver, which is configured to receive the pulses of the lightreflected from the scene so as to measure a time of flight of thepulses.
 20. A method for scanning, comprising: providing a statorassembly, which comprises a core comprising a pair of pole pieces,defining an air gap, and comprising a tooth, which protrudes between thepole pieces and has an upper end that adjoins and contains the air gap,and one or more coils comprising conductive wire wound on the core so asto cause the core to form a magnetic circuit through the air gap inresponse to an electrical current flowing in the conductive wire;providing a scanning mirror assembly, comprising a support structure, abase, which is mounted to rotate about a first axis relative to thesupport structure, and a mirror, which is mounted to rotate about asecond axis relative to the base; fixing at least one rotor, whichcomprises one or more permanent magnets, to the scanning mirrorassembly; mounting the scanning mirror assembly on the stator assemblyso that the one or more permanent magnets are positioned in the air gapso as to move in response to the magnetic circuit; and driving the oneor more coils with an electrical current at one or more frequenciesselected so that motion of the at least one rotor, in response to themagnetic circuit, causes the base to rotate about the first axis at afirst frequency while causing the mirror to rotate about the second axisat a second frequency.